To meet the ventilation needs of passenger aircraft, conditioned air is continuously supplied to the passenger cabin and flight deck. Because of the relatively large number of people that occupy a relatively small volume of space, the air in the passenger cabin is replaced approximately every two (2) minutes. The ventilation air for the passenger cabin and flight deck typically is composed of both recycled stale air withdrawn from the passenger cabin and outside air, for instance, bleed air from the aircraft engine compressor stages. Both the stale cabin air and the outside air are conditioned to a desired temperature by remotely located air conditioning units prior to delivery to the passenger cabin. Although the stale cabin air is also filtered to remove smoke and other particulate matter prior to return to the passenger cabin, in many models of aircraft, the outside air is not filtered. As a result, airport runway debris and atmospheric dust that is ingested into the aircraft engines is allowed to enter into the air distribution system of the aircraft and contaminate the passenger cabin air.
From the remotely located filters and air conditioning units, the air is distributed by ducts to various locations throughout the aircraft. Because of the limited space available in an aircraft, in cross-sectional size the air delivery ducts typically are fabricated as small as possible. As a result, the air is transmitted through the delivery ducts at relatively high pressure and speed. Flow regulators and/or restrictors are used to regulate the flow rate of the air discharged into the passenger cabin. Due to the elevated flow speed and pressure of the air in the delivery ducts, high noise levels are generated in the ducts and the airflow regulators. This noise is a large component of the cabin noise that aircraft passengers are subjected to.
Various attempts have been made to reduce the noise attendant in the supply and discharge of ventilation air to the passenger cabin. For example, sound absorbing materials, such as fiberglass and open cell foam, have been placed within air ducts and restrictors. However, lining the air ducts and restrictors with the acoustic material requires a significant amount of manual labor, and also reduces the interior cross-sectional sizes of these components thereby requiring an even faster air flow speed to maintain the same flow rate. An attempt also has been made to reduce cabin noise by the use of special noise attenuation chambers. The chambers typically are complex in design and labor intensive to fabricate and install.
Experience in the air filter art has shown that air restrictors utilizing fabric materials, such as fiberglass or NOMEX.TM., generally are of quieter operation than nonfabric restrictors constructed from, for example, metal or plastic. However, the propensity of fabric air restrictors to collect particulate matter and thus clog in service has prevented their use for airflow regulation in aircraft air distribution systems. The filters employed in conjunction with the recirculated cabin air cannot remove all airborne particulate matter, such as the airport runway debris and atmosphere dust ingested into the aircraft engines.